This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus for detecting the presence of an original document on the platen thereof.
In the process of electrophotographic printing, for example, as disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in 1942, a photoconductive member is charged to a substantially uniform potential in order to sensitize its surface. The charged photoconductive surface is then exposed to a flowing light image of the original document being reproduced. Exposing the charged photoconductive surface to the light image selectively dissipates the charge in the irradiated areas. This records an electrostatic latent image of the original document on the photoconductive surface. The electrostatic latent image is developed by bringing a developer mix of carrier granules and toner particles into contact therewith. Thereafter, the toner powder image is transferred from the photoconductive member to a sheet of support material, such as plain paper, to which it is permanently affixed by the suitable application of heat thereto. In this manner, a copy of the original document is formed. Frequently, the original document being reproduced is left on the platen of the printing machine after the copy is reproduced. Herein before, there was no indication that the operator had left the original document in the electrophotographic printing machine. Thus, when operators returned to their work stations with the copy, the original document was frequently lost.
Accordingly, it is a primary object of the present invention to improve electrophotographic printing machines by providing an indication to the operator that the original document has been left therein after the requisite number of copies have been reproduced.